This invention relates to a method of preventing cold-worked stress corrosion cracking in virtually all types of iron steels and alloyed steels including stainless steels (hereinafter, iron steels having low contents of impurity elements and alloyed steels having comparatively high contents of alloying elements are collectively referred to as “iron steels and alloyed steels”). The method comprises removing a residual tensile stressed layer, optionally generating residual compression, through the impact of irradiation with an ultra-short femtosecond pulse and kW class high average-power laser, and removing a hardened surface layer extremely susceptible to stress corrosion cracking, whereby this susceptible hardened surface layer with concentrated dislocation defects and the like is removed without generating or introducing any of such hardened layer in the process of its removal.
If iron steels and alloyed steels are cold-worked, and then are under in corrosive environments, cold-worked stress corrosion cracking occurs in its surface, as shown in FIG. 1, in which numeral 1 refers to the iron steels and alloyed steels including stainless steels as the object under corrosive environments, 2 the corrosive environments, 3 the developed residual tensile stress, 4 the susceptible, hardened surface layer which is prone to cold-worked stress corrosion cracking, and 5 the stress corrosion cracking in an advanced state.
Some knowledge has been obtained about the conditions under which cold-worked stress corrosion cracking occurs but no detailed understanding has been obtained for the mechanism by which a specific type of cracking occurs and develops. A current approach of the prevention usually relies upon retarding the chemical reaction rate of corrosion to adopt severe and strict guidelines of the corrosive environments and to control the pH of cooling water, water condensed on surfaces, and the likes.
Another current approach depends on annealing, in which the temperature of all or part of the objects are elevated so that residual tensile stress is substantially eliminated. However, this method is not applicable to specially-processed steel sections that have been forged, rolled or otherwise worked to have higher tensile strength because the required strength can not be kept after the annealing.
In another approach, selected or all surfaces of the work are impacted by steel balls or shaped-shots which are projected at high speed in a generally vertical direction and by the resulting compressive force, the residual tensile stress on the horizontal surface is converted to residual compression.
In another approach, high-power laser light is applied to selected or all surfaces of the work in water in a generally vertical direction so that they are impacted by the extinguishing pulse pressure of bubbles that form on the surface, and by the resulting compressive force, the residual tensile stress on the horizontal surface is converted to residual compression.
In another approach, a high-speed water jet is propelled to selected or all surfaces of the work in water in a generally vertical direction so that they are impacted by the extinguishing pulse pressure of the cavitation that develops on the surface, and by the resulting compressive force, the residual tensile stress on the horizontal surface is converted to residual compression.
In another approach, a hardened surface layer is roughly removed from a selected area or all areas of the work by means of an abrasive such as diamond powder, sand particle and others with the ground area or areas and the surrounding areas being cooled to ensure that no heat generation occurs or no additional hardened layer will form, and the remaining hardened layer is etched away in an acidic solution with or without an applied electric field. If desired, either the grinding step or the etching step alone may be performed. However, it is difficult to achieve uniform processing by this method since the grain boundaries in the work are etched irregularly in larger area.
Another method takes an entirely different approach and manufactures a work from a specially designed material such as low-carbon stainless steel which has been rendered less sensitive to stress corrosion cracking in that stress corrosion cracking will not easily develop or progress.
These methods that are intended to prevent cold-worked or otherwise induced stress corrosion cracking have some field-proven records; however, the type of work that can be processed, the place where processing can be done, the shape of the work, and other conditions are so rigorous that the future and current applicability of the methods is fairly limited and their effectiveness and effective periods are also limited to cause the following problems.
Ever since the beginning of recorded history, cold-worked stress corrosion cracking has put considerable limit on the performing functions and serviceable periods of many tools, machines, vehicles and structural parts made of the iron steels and alloyed steels including stainless steels, and on account the difficulty in predicting it, cold-worked stress corrosion cracking has been a cause of many cracking accidents.
Even today, cold-worked stress corrosion cracking contributes to markedly increasing the production and maintenance costs of many nuclear reactors, power plants, production facilities in heavy chemical industries, bridges, buildings made of the iron steels and alloyed steels including stainless steels, aircrafts, automobiles, railways and all other structures made from the iron steels and alloyed steels including stainless steels. Since cold-worked stress corrosion cracking limits the performance and functions and serviceability periods of those structures and the timing of cracking is difficult to predict, this type of cracking has been a cause of many accidents.
Environmental factors such as the water content and the corrosive ingredients in water have been dealt with by pH control and other methods; however, not all of them can be controlled by manufacturing technologies and there is a need to solve the problem of cold-worked stress corrosion cracking by developing an ideal manufacturing technology which, even in the presence of some environmental factors, can control the other two conditions so as to overcome the horrible environments.
The approach that relies upon retarding the development of corrosion by adopting strict guidelines for corrosive environments as for the pH of water and concentration of the chlorine ingredient in it is not usually applicable at all times. Depending on the conditions of the apparatus, the method is not usually applicable in many cases.
The approach which performs annealing to elevate the temperature of all or part of the work so that residual tensile stress is substantially eliminated throughout the work ranging from the surfaces to the interior is difficult to apply to large or complex shapes; it is not applicable at all if the object has thermally vulnerable parts or areas; it is also difficult to apply on-site to parts or apparatuses that require high rigidity or strength. Thus, the approach under consideration cannot always be adopted and more often than not it is generally unsuitable for use.
The approach in which selected or all surfaces of the work are impacted by steel balls or other shaped shots which are projected at high speed in a generally vertical direction and by the resulting compressive force, the residual tensile stress on the horizontal surface is converted to residual compression, is not applicable in all cases and the effect lasts for only a short peroid, thus requiring repeated processing. In addition, it often occurs that interference by nearby equipment precludes application of the approach. As a further problem, scattering steel balls or other shaped shots often impairs the integrity of equipment, making the approach unsuitable for use in circumstances where they are difficult to recover.
The approach in which high-power laser light is applied to selected or all surfaces of the work in water in a generally vertical direction so that they are impacted by the extinguishing pulse pressure of bubbles that form on the surface, and by the resulting compressive force, the residual tensile stress on the horizontal surface is converted to residual compression, is difficult to apply to very large structures, miniature structures and complex shapes; it is impossible to apply if the work has parts or areas that cannot be put into water; it is also difficult to apply in places that dislike the use of water. Thus, the approach under consideration cannot always be adopted and more often than not it is generally unsuitable for use.
The approach in which a high-speed water jet is propelled to selected or all surfaces of the work in water in a generally vertical direction so that they are impacted by the extinguishing pulse pressure of the cavitation that develops on the surface, and by the resulting compressive force, the residual tensile stress on the horizontal surface is converted to residual compression, is difficult to apply to very large structures, miniature structures and complex shapes; it is impossible to apply if the work has parts or areas that cannot be put into water; it is also difficult to apply in places that dislike the use of water. Thus, the approach under consideration cannot always be adopted and more often than not it is generally unsuitable for use.
The approach to be considered next is one in which a hardened surface layer is roughly removed from a selected area or all areas of the work by means of an abrasive such as diamond powder, sand particle and others with the ground area or areas and the surrounding areas being cooled to ensure that no heat generation occurs or no additional hardened layer will form, and the remaining hardened layer is etched away in an acidic solution with or without an applied electric field. If desired, either the grinding step or the etching step alone may be performed. However, these methods are difficult to apply to very large structures, miniature structures and complex shapes; they cannot be applied if the object has parts or areas that are vulnerable to acidic corrosive chemicals; they are also difficult to apply on-site. Thus, the methods under consideration cannot always be adopted and more often than not they are generally unsuitable for use.
Speaking of the method which manufactures a work from a specially designed material such as one in which stress corrosion cracking will not easily develop, or one which has been rendered less sensitive to stress corrosion cracking, or one in which stress corrosion cracking will not easily develop, no such materials have been developed and many iron steels and alloyed steels including stainless steels still remain unsuitable for use, so it is impossible to apply the method in almost all cases. The few developed, new materials are so expensive that the method is not applicable in all cases for economic and other reasons. More often than not the method is generally unsuitable for use.